The invention concerns a method for preventing fouling and sheeting in polymerization reactors, particularly in gas phase reactors of multiphase polymerization processes.
Various methods for manufacturing solid polymers from hydrocarbons, for example from 1-olefins have been developed. In one such method olefins, such as ethylene, propylene or butene, are polymerized in the presence of catalysts in hydrocarbon diluents or in monomers acting as diluents. The reactants are kept in liquid phase by maintaining a proper pressure in the polymerization reactor. When the polymer is insoluble or only slightly soluble in the diluent, the polymer product forms as particles and therefore the product flow consists of a suspension formed by polymer particles, diluents and monomers.
One reactor type applied in such methods is a continuous pipe reactor forming a loop, where the polymerization is carried out in a circulating turbulent flow. The product containing polymer, diluent and monomers, is taken from the loop reactor either continuously, or more usually, periodically through a discharge valve and it is introduced to a separator, where the polymer is separated by lowering the pressure.
Another reactor type applied in such methods is a gas phase reactor, where polymerization is carried out in the presence of catalysts and gaseous monomers. Typically the polymerization is carried out in fluidized bed reactors, where polymerization is carried out in a bed formed by polymerizing polymer particles. This bed is kept in fluidized state by circulating gaseous flow from the top of the reactor to the bottom of the reactor. Polymerization heat is removed by cooling said circulating gaseous flow.
It is also known continuous multiphase processes, where slurry reactors, such as loop reactors are followed by one or more gas phase reactors.
One problem, which frequently is found especially in loop reactors, is the adhering of the polymer particles onto the reactor walls. Even slight amounts of polymer cause that the smoothness of the reactor inside surface disappears, whereafter the adhering starts to accelerate and causes in the worst case the blocking of the reactor. A polymer layer on the reactor surface actually increases the flow resistance of the polymer suspension and the pumping power required. At the same time die heat transfer efficiency of the reactor decreases and makes the temperature control more difficult. If high polymerization temperatures are used, the result can be melting of the polymer.
Also in gas phase reactors similar problems can be found. The polymer particles tend to adhere to the reactor surfaces and reactor internals and also polymer particles tend to adhere to each other and form lumps. This will cause flow disturbances in the fluidized bed and local overheating because polymerization heat cannot be effectively removed from larger polymer lumps. Especially polymer sheets adhering on reactor walls and later detaching cause severe problems in the product quality.
The harmful fouling phenomena described above has been tried to be avoided by adding into diluent antistatic agents, which make the diluent more conductive and thereby prevent at least somewhat the formation of Static electric charges. However these kind of antistatic agents are usually harmful for polymerization catalyst, because they act at least in some degree as catalyst poisons and thereby decrease catalyst activity. Therefore, the amounts of known fouling preventive agents must generally be rather low. Similarly various antistatic compounds have been tried also in gas phase reactors to decrease sheeting problems.
In U.S. Pat. No. 3,956,252 it has been proposed as antistatic agent a nitrogen containing salt of phytic acid or a mixture of it with an alkali metal salt of an organic acid.
In U.S. Pat. No. 3,995,097 it has been proposed as antistatic agent a mixture of an aluminum or chromium salt of an alkyl salicylic acid and an alkali metal alkyl sulfosuccinate.
In U.S. Pat. No. 4,012,574 it has been prosed as antistatic agent a surface-active compound, which contains one or more perfluorocarbon groups.
In U.S. Pat. No. 4,068,054 it has been proposed as antistatic agent a porphyrine compound either alone or together with a metal alkyl sulfosuccinate.